Automatic transmission band control circuit



D. E. FOSTER AUTOMATIC TRANSMISSION BAND CONTROL CIRCUIT Filed June 27, 1956 2 Sheets-Sheet 1 sept. 21, 1937.

Sept. 21, 1937. Y D. E. FOSTER 2,093,555

K AUTOMATIC TRANSMISSION BAND CONTROL CIRCUIT Filed June 27, 1936 ZSheets-Sheet 2 INVENTCR DUDLEY E. FOSTER ATTORNEY Patented Sept. 21, 1937 j maar AUTOMATIC TRANSMISSION BAND CON- TROL CIRCUIT Dudley E. Foster, Morristown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 27,

7 Claims.

My present invention relates to` control circuits for regulating the signal transmission band characteristics of a radio receiver, and more particularly to a signal-responsive arrangement for automatically modifying the signal transmission band characteristic of a receiver.

As is well known, dissymmetry of the transmission band characteristic of a radio receiver about the normal, or mean, frequency value produces a relative phase shift of the modulation side bands. For example, in a superheterodyne receiver the result of dissymmetry of the overall I. F. network transmission band characteristic is `distortion caused by the relative phase shift of the modulationside bands of the I. F. energy. The lack of side band symmetry may be caused by any one of the following phenomena. There may be spurious couplings between the resonant networks of the cascaded'transmission circuits; regenerative effects may give rise to dissymmetry of the transmission band. Again, overall mistuning of` individual I. F. stages may also give rise to the dissymmetry referred to. Regardless ofthe cause,

or causes, of the'dissymmetry, this lack of symmetry of sidebands is undesirable duces detector distortion.`

Now I have found that such transmission band dissymmetry, when occurring, may be substantially eliminated by deliberately introducing into the signal transmission patha compensating effect which involves' thel introduction of a complementary dissymmetry; ,theloverall effect of the two dissymmetries being-such that the resultant signal transmission lband characteristic is symmetrical with respect to a predetermined mean frequency value. In addition, I have found that the compensating dissymmetry can be introduced into the signal transmission path in response to thereceived signal energy, and that the compensating dissymmetry may be caused to appear on either side of the predetermined mean frequency value of the transmission band characteristic.

Accordingly, it may be stated `that it is one of the main objects of my present invention to provide a receiving system which includes a signal transmission path of Vatleast three resonant circuits "all tuned to a common operating signal frequency; the second and third of the resonant circuits being cppositely unsymmetrical on equal sides of the operating `frequency, whereas the rst resonant circuit has a resonance curve characteristic which `is symmetrical with respect to -the operating frequency; there being, additionally, provided a controlarrangement which is consince it pro- 1936, Serial No. 87,615

(Cl. Z50-20) structed in such a manner that when the band characteristic of the first `resonant circuit becomes unsymmetrical, the following resonant circuits have their signal transmission efficiencies relatively altered in a sense such that the dissymmetry in the rst resonant circuit is substantially compensated for.

Another important object of the invention is to provide an I. F. network for a superheterodyne receiver, which network includes at least three cascaded circuits resonated to the operating I. F., the first of the I. F. circuits having a symmetrical resonance curve characteristic, whereas the second and third I. F. circuits have resonance curve characteristics which are opposite-unsymmetrical with respect to the operating I. F., and a pair of I. F. rectier circuits being provided with tuned circuits whose resonance curve characteristics correspond respectively to the second and third I. F. circuits; gain control connections being provided between the rectifier circuits and corre'- spondingI. F. circuits so that when the characteristic of the rst I. F. circuit becomes unsymmetrical, the rectier circuits will function to control the relative I. F. transmission eiciencies of the second and third-I. F. circuits in a sense to compensate for the lackof symmetry in the characteristic ofthe first I. F. circuit.

Another object ofthe invention may be stated to reside in the provision in a`transmission band o modifying arrangementv ofthe type described in the preceding paragraph, of an automatic volume control arrangement which `functions to vary the transmission eiiiciency of the I. F. circuit whose r characteristic is symmetrical, and wherein suchy volume control action depends upon the average value of the control bias produced by the pair of I. F. rectier circuits; j

` Stilllother objectsofl the invention are to improve generally the efficiency of operation of radio receivers of the superheterodyne type, and moresespecially to provide an I. F. transmission band characteristic modifying arrangement, for a superheterodyne receiver, which will not only be reliable in operation, butbe economically manuiactured and assembled in a receiver. The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the `following description taken in connection with the drawings in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawings,

Fig. 1 shows a circuit diagram of a portion of the superheterodyne receiver embodying the present invention,

Fig. 2 illustrates graphically the various resonance curve characteristics of the networks forming a part of the invention,

Fig. 3 is the overall resonance curve characteristic of the I. F. transmission network of the receiving system,

Fig. 4 is an unsymmetrical resonance curve characteristic which the present invention is designed to correct.

Referring now to the accompanying drawings, wherein like reference characters in the different gures designate similar elements, there is shown in Fig. 1 the portion of a superheterodyne receiving system disposed between the output of the first detector and the input of the audio frequency amplifier network, the transmission band characteristic modifying arrangement, as Well as the automatic volume control circuit, being embodied in this portion of the receiving circuit. In a superheterodyne receiver of the type shown in Fig. l, the I. F. amplifier l, shown as of the pentode type, usually has connected between its input electrodes a resonant network which is tuned to the operating I. F. Such a network is shown at T1; it will be seen that it comprises a transformer whose primary and secondary windings 2 and 3 are respectively tuned to the said I. F. value. As is well known to those skilled in the art, each of these windings is tuned by a condenser, and the tuned primary 2 may be disposed in the output circuit of the first detector.

The circuit elements preceding the tuned primary 2 may comprise the usual signal collector; one, or more, stages of tunable radio frequency amplification; the amplified radio frequency signals being fed to the conventional type of first detector. Of course, a tunable local oscillator is provided; and a common tuning control device is provided, for varying the tuning of the radio frequency amplifiers, first detector and local oscillator in such a manner that the signal energy produced in the circuit 2 is constantly of the operating I. F. value. If desired, the first detector and local oscillator may utilize a 2A7 type tube connected to provide an electron coupled converter stage. Assuming that the receiving system is employed in the broadcast range of 500 to 1500 k. c., the operating I. F. may have a value of 460 k. c., although it may vary in value from 75 to 460 k. c. In addition, the receiver may be of the multi-range type.

The amplifier l has the usual signal grid biasing network 4 disposed in its grounded cathode lead, and in its plate circuit is disposed the tuned primary circuit 5 of the following I. F. coupling transformer T2. The tuned secondary circuit 6 of transformer T2 is connected between the input electrodes of the I. F. amplifier 1, and this. ampliiier includes in its cathode lead the signal grid biasing network 4. A third I. F., amplifier 8, including the signal grid biasing network 4" in its cathode lead, has its input electrodes connected to opposite sides of the tuned secondary circuit 9 of the I. F. coupling transformer Tx. The tuned primary circuit 2) of transformer T3 is disposed in the plate circuit of I. F. amplier l. The second detector is shown as of the diode type and is denoted by the reference numeral ll, the anode of the diode l I being connected to the high alternating potential side of the tuned secondary circuit l2 of I. F. coupling transformer T4. The cathode of diode il is grounded, and is connected to the low alternating potential side of tuned secondary circuit l2 through the diode load resistor i3, the latter being shunted by the I. F. bypass condenser lll. The tuned primary circuit i5 of transformer T4 is disposed in the plate circuit of the I. F. amplifier 8.

The audio frequency component of rectified I. F. current flowing through the load resistor I 3 is transmitted to the audio frequency network through the audio condenser I6, and it will be understood that the audio network may comprise one, or more, stages of audio frequency amplification, followed by a reproducer of any desired type. It will be understood that each of circuits 2, 3; 5, 6; lo, 9; i5 and l2 are tuned to the operating I. F. of 460 k. c. However, the resonance curve characteristics of the stages including coupling transformers T1, T2, T3, and T4 are different. In Fig. 2 there are shown the various resonance curve characteristics of the individual I. F. coupling transformers, it being understood that each of the characteristics is that of a coupling transformer in conjunction with its immediately preceding tube. Thus, the resonance curve characteristic of the 'transformer T1 and the tube immediately preceding it, that is` the first detector tube, is shown as the first curve of the series of curves.

The full line curve is a double peaked curve of the band pass type, and is secured, as is well known, by employing coupling between windings 2 and 3 which is greater than critical. The dotted line curve is of the single peak type, and may be the characteristic of the transformer T1 when it is desired to have this transformer provided with a single peak characteristic. Such a single peak characteristic is secured by coupling windings 2 and 3 with critical coupling. The next coupling transformer T2 has a resonance curve characteristic similar to that of transformer T1. Hence the full line curve is exactly the same as the full line curve of transformer T1, and there is also shown, in dotted line, the single peak characteristic for the transformer T2. Here, again, it is to be understood the characteristic shown is that of the transformer T2 considered in conjunction with amplifier l. transformer T3, considered in conjunction with amplifier l, has the resonance curve characteristic shown by the third full line curve in Fig. 2. It will be seen that this curve is unsymmetrical with respect to the I. F. value, and that it is prominently peaked on the minus, or lower, modulation side band.

On the other hand, the transformer T4, considered in conjunction with amplifier 8, has a resonance curve characteristic illustrated by the fourth full line curve in Fig. 2; it will be observed that this characteristic is unsymmetrical with respect to the I. F. value in the opposite sense of the characteristic of transformer T3. In other words, the coupling transformers T3 and T4 have transmission band characteristics such that they favor the lower and upper modulation side bands of the I. F. carrier respectively. These last two characteristics may be secured in any well known manner. For example, the condenser l0 of tuned circuit i t may be made adjustable so as to detune circuit l@ a sufficient frequency distance from the operating I. F. Value so as to produce the dissymmetry shown in Fig. 2. In the same way the condenser i5 may be made adjustable to The coupling il A l i `secure the dissymmetry shown in the `fourth char- `acteristic `curve of Fig. 2. Additionally, and `in A i OIT.

jplace of the detuning method of securing dissyrnmetry, the distributed'cap-acity, shown in dotted lines and denoted by the numeral 9', between coils 9 `and lilmay be used to secure the dissymmetry in the characteristic curve of transformer Similarly, the distributed capacity l2@ between'coils lZfand'IS may be used to secure the dissymmetry in the characteristic curve of trans-` former T4. e Y

` In Fig. 3 there is shown the overall resonance e. curve characteristic of the signal transmission path between the .kiirst detector and thesecond detector. The full line curve in Fig. 3 is obtained by combining the rst four full line characteris- The dottedl line portion or the gle peak characteristics. l It will be observed that the overall characteristic in Fig. 3 is symmetrical,

` and when this is the nature of the characteristic the I. F. energy delivered to the second detector 25` will not cause detector distortion. When this overall characteristic becomes unsymmetrical, detector distortion results. l

` It is pointed out that the band widths of the curves T3 and T4 are approximately i4'.` k. c., 30

whereas the bandwidth i' the characteristic `curves of transformers T1 and T2 are shown as 10 k. c. For this reason the overall characteristic valuesare subject to change depending upon the to illustrate that the band Width of the compensating transformers `need not necessarily be i the'same as that of the normaltransforrners of i the circuit. In Fig. 4 there is shown the charac- 40 teristic curve of the coupling transformer T1 and/or T2 when dissymmetry occurs.` Letit be assumedthat for any of the reasons given heretoforeatleast one of the characteristiccurves of the stages including transformers T1 `and'Tz l becomes unsymmetrical with respect totheoperating I. F. 'Ihis, without any corrective or modi- `tying action', produces a dissymmetry of side band transmission inthe overall I. F. band transmis- Ysion characteristic. In other words, the curve inFig. 4 illustrates the resultant characteristics of the stages including transformersTi and T2- when dissymmetry` in at least one of these two characteristics takes place. When this happens more energy will reside in frequencies lower than the center frequency and, therefore, more lower side band energy will be'deliveredto the detector thus resulting indetcctor distortion. The control circuit shown in Fig. 1 is constructed and arranged tocorrect for, and eliminate, therdissymmetry shown in Fig. 4.' Atthe same time that this is accomplished, normal AVC action to vcorrect for signal fading is secured.

The control circuit comprises a tube il of the duplex-diode-pentode type, commercially known as a 2B? type tube. 'I'he signal grid I8 of this tube is connected through the I. F. bypass condenser l9 tothe plate side of the tuned primary `circuit l5. A The grounded cathode lead of tube Il includes the usual biasing network 2l) for the input grid i8, andwthe latter` is connected to ground through theresistor 2i. TheV plate il of tube il is connected to a source of positive potential through the resonant circuit 2l, the latter being fixedly tuned to the operating I. F. value; The coil 22 of circuit 2| is theprimary a pair of diode sections.

velement for this gain control connection.

areprovidedtwo secondaries 23` and 24.` The secondary winding 23 has connected across it the condenser 25, and the secondary winding 24 hasjconnected across it condenser 2B. `The coil 22 is magnetically coupled toV coil v23, and the coil 2 2 is also magnetically coupled to the coil 24. However, coils 23 and24 have zero coupling between them, and it is to be understood that there is Yto be no coupling from coil 23 to coil 24 throughv the coil 22.` To accomplish this any means well known to those skilled in the art may beprovided inthe primary circuit 2| in order to maintain zero coupling between `coils 23 and 24.

`Each of condensers 25 and 26 is made adjust#` ableso that it can be adjusted to detune the inputcircuits oi the diode rectiers on either sideof the operating I. F. The tube Il includes `Those skilled in the art are fully aware of the construction of a tube of the 2137 type, and know that the tube includes 32. 'I'he diode anode 40 is connected to the cathode 3l through a series: path which comprises secondary coil 24 and the diode load resistor 4I,

in Fis. 3 has a Width 0f 1,0 k- G- Of Course. these the resister M being shunned by the I. F. bypass condenser 42.

52 is connected to ground from the lead 50. The

function of resistor i and condenser 52 is to lter out the pulsating component `from the direct current voltage which is transmitted through lead'il to the signal input grid of amplifier 8.

In the same manner the lead 60 is connected between the anode side of the load resistor 3l' and the low alternating potential side of the tuned secondary circuit 6. The numerals 6I and |32 denote the pulsating component lter The normal AVC connection for the receiver circuit is designated by the letters AVC, and this comiprises the lead 'iti which is connected tolead 5l] through resisto-r il, and to lead E!) through resistor l2, the junction of` resistors 1I and 12 `neing connected to ground through the con-y denser 13.

The resistors ll and 12, as Well as condenser 13, function to suppress the pulsating component of the direct current voltage transmitted through lead 'lli to the signal grid circuit of am.- plier l. The lead 1o may, also, be connected to any of the tubes preceding the I. F. amplifier I,`

and, for example, this AVC connection may be lmade to the tunable radiolfrequency amplifiers and the first detector signal grid, if desired. It will be appreciated that the direct current voltv age which is transmitted through lead is the average of the direct current voltages derived from across the resistors 3| and 4l. Whereas pliiiers l and 8 depends directly upon the direct coil of the coupling transformer T5, andthere` current voltage developed across resistors 3| and 4| respectively.

The resonance curve characteristic of the secondary circuit S1, the input to the diode rectifier 3!-3!, has a resonance curve characteristic which is illustrated by the fifth full line curve of Fig. 2. It will be noted that this characteristic corresponds to the characteristic of coupling transformer T3. On the other hand the resonance curve characteristic of the secondary circuit S2, the input circuit of diode rectifier 3 I-Ml, is illustrated by the last full line curve of Fig. 2. This characteristic of secondary circuit S2 corresponds to the resonance curve characteristic of transformer T4. The resonance curve characteristic of circuit S1 may be produced by adjusting condenser 25 to detune the circuit S1 sufficiently to secure a dissymmetry such that the lower modulation side band is favored; the condenser 255 of circuit S2 is adjusted to secure the characteristic wherein the upper modulation side band is favored. It is pointed out, in addition, that in each of the last four characteristic curves of Fig. 2 the spacing between the pronounced peak in its particular side band from the I. F. value is approximately k. c.

In considering the operation of the system shown in Fig. l, let it be assumed that for one reason or another dissyrnmetry of the type shown in Fig. 4 occurs. Then more signal energy will reside in frequencies on the side of the minus, or lower, modulation side band, and hence more signal energy will exist in the input circuit S1, since its characteristic in Fig. 2 shows that it favors the lower modulation side band. Accordingly, more direct current voltage will be cleveloped across resistor 3l' than across resistor 4|. This means that more negative bias will be transmitted through lead @il to the signal grid of amplifier l, and therefore reduce the gain of the amplifier. By the same token the bias transmitted through lead 5i? to the signal grid of amplil'ier 8 will decrease, and the gain of amplifier 8 will increase with respect to that of amplifier 1. In other words, the amplifier feeding coupling transformer T3, whose characteristic favors the lower modulation side band, decreases in gain, whereas the amplifier feeding transformer T4, whose characteristic favors the upper modulation side band, increases in gain. This results in a compensation of the dissymmetry shown in Fig. 4.

It will now be seen that the corrective action of thc automatic control bias derived from the two diode rectifier circuits will vary the differential gain of T3 and T4 so that a symmetrical transmission characteristic is produced. If the uncorrected transmission characteristic were such that the upper, or positive, modulation side band were accentuated in the overall transmission curve, relatively more gain control bias would be developed in the diode circuit connected to circuit S2. This bias would be applied to amplifier il to decrease the energy transmission of the upper modulation side band, and concurrently increase the lower side band amplitude.

The normal AVC connection is derived, as stated above, from b'oth diode rectiiers, and is applied to amplier l which feeds the transformer T2, as well as the tube feeding the transformer T1. The bias voltage transmitted over lead lil is the average, or mean, value of the direct current voltages derived from resistors 3l and 4|. As the received signal amplier increases, the amplitude of the signal energy increases thereby increasing the voltage drops across resistors 3| and 4|. 'I'his results in an increase in the mean value of both voltages, and a resultant decrease in gain of amplifier l as well as any of the preceding tubes of the receiving system.

The control circuit is shown separate from the audio detector, and this arrangement is employed since it permits a` symmetrical transmission characteristic, with resultant freedom from distortion, 1

to be applied to the audio detector. The audio frequency current may also be obtained from the same pair of diodes which supply the AVC voltage. In this case one of the diode rectiflers will introduce audio frequency distortion of one phase, and the other diode will produce audio frequency of equal magnitude, but opposite phase. In this way the two audio frequencies will tend to cancel. Hence, if a separate audio detector Il is not desired for design reasons, the audio output of the two diodes may be employed to supply the audio currents for the audio amplification network.

It will now be seen that there has been provided a signal rectifier system which functions to maintain the overall I. F. characteristic shown in Fig. 3, in spite of dissymrnetries of the type shown in Fig. fi. As soon as a dissymrnetry of the latter type occurs, the signal rectifier network acts to change the relative transmissions through the networks T3 and Ti so as to restore the symmetrical characteristic shown in Fig. 3. At the same time, the normal AVC action is secured from the saine control circuit by utilizing the average value of current voltages produced by the two signal rectiflers.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular circuit organization shown and described, but that many modifications may be made without departingl from the scope of my invention, as set forth in the appended claims.

What is claimed is:

l. In combination in a signal transmission system comprising at least two resonant circuits, each of the circuits being tuned to an operating side band modulated carrier wave frequency, the resonance curve characteristic of one of said resonant circuits being symmetrical with respect to the mean frequency value of the characteristic, the resonance curve characteristic of the other resonant circuit being unsymmetrical with respect to the mean frequency value of the characteristic, a signal rectifier having an input circuit tuned to said operating frequency and having a resonance curve characteristic which is unsymmetrical with respect to the mean frequency value, and means, responsive to the uni-directional current output of the signal rectiferfor varying the signal transmission through said rst unsymmetrical resonant circuit in a sense such as to compensate for an undesired dissymmetry in the resonance curve characteristic of the said symmetrical resonant circuit.

2. In a superheterodyne receiver provided with an I. F. transmission .network comprising at least three resonant circuits arranged in cascade, each of said circuits being tuned to an operating I. F. value, the resonance curve characteristic of the first tuned circuit being symmetrical with respect to its characteristic mean frequency value, the resonance curve characteristic of the second tuned circuit being unsymmetrical in one of the modulation side bands with respect to the characteristic mean frequency value, the resonance curve characteristic of the third tuned circuit being unsymmetrical in the opposite sense from the dissymmetry of the second circuit, a pair of signal rectiers arranged to receive I. F. energy from the said I. F. transmission network, one of said rectiers having an input circuit provided with a characteristic corresponding to the characteristic of the second I. F. tuned circuit, and the other rectifier circuit having a tuned input circuit whosecharacteristic corresponds to the characteristic of the third I. F. tuned circuit, and means for utilizing the direct current voltage output of the signal rectiers to vary the relative transmission e'iciencies through the second and third I. F. tuned circuits.

3. In a superheterodyne receiver provided with an I. F. transmission network between the output of the rst detector and the input of the second detector, at least three resonant circuits arranged in cascade, each of said circuits being tuned to an operating I. F. value, the resonance curve characteristic of the rst tuned circuit being symmetrical with respect to its characteristic mean frequency value, the resonance curve characteristic of the second tuned circuit being unsymmetrical in one of the modulation side bands with respect to the characteristic mean frequency value, the resonance curve characteristic of the third tuned circuit being unsymmetrical in the opposite sense from the dissymmetry of the second circuit, a pairV of signal rectiers arranged to receive I. F. energyfrom the said I. F. transmission network, one of said rectiers having an input circuit provided with a characteristic corresponding to the characteristic of the second I. F.

tuned circuit, and the other rectiiier circuit having a tuned input circuit whose characteristic corresponds to the characteristic. of the third I. F. tuned circuit, means for utilizing the direct current voltage output of the signal rectiers to vary the relative transmission efciencies through the second and third I. F. tuned circuits, and

means for utilizing the mean direct current voltage output of the two rectiers for varying the l signal transmission efficiency through the rlrst I. F. tuned circuit.`

4. In a superheterodyne receiver provided with l an I. F. transmission network, at least two amplifying stages in cascade, one of said stages being dissymmetrical about the operating I. F. so that one modulation sideband is increased relative to the other modulation sideband, the second of said stages being dissymetrical in the opposite sense so that the two in cascade produce a symmetrical transmission characteristic, a pair of signal rectiiiers with their associated input circuits so arranged that if the applied frequency is higher than the operating I. F. more energy is supplied to the one rectierthan the other, whereas if the applied frequency is lower than the operating I. F. more energy is supplied to the second rectier, means for applying the direct current voltage output of each rectirler to one of said amplifying stages, whereby the gain of the amplifying stage having dissymmetry in one sense, with respect to the operating frequency, is

reduced when additional .energy on that same side of the operating frequency is applied to the signal rectier associated therewith.

5. In a system as defined in claim 1, and additional means, responsive toy the direct current voltage output of the rectifier, for decreasing the signal transmission through the symmetrical resonant circuit as the carrier wave amplitude increases.

6. In a system as defined in claim 2, means independent of the signal rectiers coupled to the last of the cascaded resonant `circuits for demodulating the I. F. energy.`

7. In a superheterodyne receiver provided with an I. F. transmission channelhaving at least two amplier networks in cascade, the resonance curve characteristics of the networks being oppositely unsymmetrical about the operating I. F. whereby the networksA in cascade produce a sym-V metrical resultant transmission characteristic, at least two rectiers, means for applying I. F. en-

ergy from the channel output to the rectifiers, l

the input circuits of the rectiers having resonance curve characteristics which correspond to` said two unsymmetrical characteristics, and a gain control connection between each rectifier 

